1. Field of the Invention
The present invention generally relates to a vibration control system for a body or an object which is supported on a movable base (also referred to as foundation or groundwork) and which is to be isolated from vibration. More particularly, the invention is concerned with a vibration control system for actively reducing the vibration of a body to thereby ensure a great reduction of the vibration against which the body is to be isolated.
2. Description of the Concerned Art
In conjunction with the vibration absorbing or damping structures for reducing vibrations and noise transmitted from prime movers, driving machines or the like to the environment, or protecting cargo on board in the course of transportation by trucks or freight cars from vibration or preventing vibration from being transmitted to an electron microscope or precision apparatus such as integrated circuit (IC) manufacturing equipment and others, it is known to support the body with a vibration isolator to be protected from the vibration by means of an elastic member constituted by a vibration isolating rubber, metallic spring or the like and/or by interposing a damper, as the case may be.
FIG. 8 of the drawing shows schematically one of the typical one of the vibration isolating supports known heretofore, and FIG. 9 illustrates the frequency characteristic of vibration transmissibility (vibration response ratio) of the structure shown in FIG. 8.
Referring to FIG. 8, the vibration damping support known in the art is implemented in a passive structure in the sense that a body 1 to be protected against vibration which is transmitted from a vibration source or foundation 3 is supported by an elastic member 2 having a spring constant K and a damping coefficient C and interposed between the body 1 and the base or foundation 3. With such a supporting structure, however, a maximum vibration response, i.e. resonance takes place at a frequency f.sub.O determined in accordance with f.sub.0 =.sqroot.K/M/2.pi. (where M represents the mass of the body 1). The result of which is that the vibration transmitted to the body 1 is not damped but undesirably amplified at or in the vicinity of this frequency f.sub.O, involving a more serious situation, to great disadvantage.
Under the circumstances, there has been recently proposed and adopted in practical applications an elastic supporting structure in which the spring constant K and the damping coefficient C of the elastic member 2 are rendered variable or adjustable (i.e. a so-called semi-active support).
However, the semi-active support can not avoid the occurrence of resonance when the vibration originating in the base or foundation includes various frequency components over a wide frequency range or band, as exemplified by random vibration. In other words, when the frequency band of the vibration of concerned is broad, there exists unavoidably a frequency at which resonance can take place. Consequently, the vibration tends to be amplified rather than damped at or in the vicinity of the resonance frequency, whereby the body is subjected to intensive vibration, giving rise to a serious problem.
Thus, with the passive or semi-active vibration damping structure known heretofore, it is impossible or very difficult to realize effectively the reduction of vibration over a wide frequency range in a satisfactory manner.